Liquid-integration already at the microfabrication stage is a new enabling feature for MEMS with applications to life-science devices, high-sensitivity sensors where the liquid state can enhance sensing capabilities, and MEMS lenses. Specific examples include MEMS-based drug delivery systems which are prefilled with drugs or protein solutions, liquid-based electrochemical sensors using liquid electrolytes which enable sensitivity levels in the ppb-range and miniaturized optical lenses. The difficulties related to liquid integration in MEMS often relate to the diminished temperature budget in processing which occurs when the liquid is integrated. This can be as low as 37° C. for sensitive or living materials in for instance life-science applications. In traditional wafer-level integration schemes the liquid is hermetically plugged inside the cavity during the wafer bonding process. This forces the cavity formation and sealing, i.e. wafer bonding, to use room temperature processes, specifically adhesive wafer bonding.
Previously there has been used wafer level room temperature hermetic liquid sealing by gold ring embossing, where a smaller gold ring on one wafer is compressed towards, and partially embedded in, a larger gold ring on the other wafer. There has also been shown cold welding of overlapping gold sealing rings with negative-slope sidewall angles. Both these methods seal the liquid in the cavity during wafer bonding and require additional mechanical stabilization afterwards. This was implemented using polymer underfills. These methods have the advantage that they can be more hermetic than pure adhesive wafer bonding since the seal is metallic instead of polymeric. A potential limitation is the fact that the liquid was pipetted into every cavity using a serial process. This was recently addressed using a method of cavity formation and cavity sealing with the wafers submerged in the liquid to be integrated. This method is however unsuitable when compared to serial pipetting for some applications. These applications could be integrating solutions which may pollute or alter the surface it comes in contact with, for instance surface fouling proteins, or when there is a risk of contaminating the liquid from the “immersion bonding” process itself. The pipetting method also has the advantage of being able to integrate two liquids, a necessity for the previously mentioned lens.
Thus there need in packaging of MEMS (Micro Electro-Mechanical Systems) devices for providing them with hermetic packages, either to protect their structures from harsh external environments and/or to ensure special atmospheric conditions or temperature sensitive liquids inside the package to ensure the functionality of the device.
Wafer-level bonding techniques are widely used for this purpose. Conventional techniques like fusion bonding, compression bonding, anodic bonding or eutectic bonding need high temperatures, high pressures, high voltage or special surface conditions. Such techniques contain process incompatibilities with many MEMS devices and are not compatible with standard microelectronic manufacturing processes. The hermetic sealing of MEMS devices and/or microelectronic circuits according to the state of the art give high manufacturing costs.
A lot of effort has been invested to develop bonding methods compatible to standard microelectronic processes. Newer techniques such as localized heating bonding require complicated structures and manufacturing steps. The sealing of the cavities with solder bonding does affect the atmosphere inside the cavity. The use of low temperature adhesive bonding techniques does not achieve hermetically plugged cavities.
Therefore, a technology is needed to hermetically plug cavities by wafer or chip level sealing of cavity holes to seal gas, vacuum or liquid filled the cavities at low temperatures.